APELLA: Open-Source, miniaturized All-in-One powered Lab-on-a-Disc platform

Graphical abstract

(ULar, China) and four lightweight and high-efficiency motor-propeller units from a mini-quadcopter (YuuHeeER, China). Rather than utilizing a metallic material, the APELLA framework is constructed from lightweight polymethyl methacrylate (PMMA), measuring at a thickness of 1.6 mm. The PMMA not only reduces rotational inertia but is also easy to machine. Fig. 2 shows a photo ( Fig. 2A) and an exploded diagram of components (Fig. 2B) in a rotational part of the APELLA. The bottom Qi receiver powers the Arduino nano microcontroller, the Bluetooth 2.4 GHz transmitter module, electronic modules, and the Wi-Fi spy camera. Another Qi receiver on top of the framework powers the motor driver board to drive the motor-propeller based propulsion mechanism. The Arduino reads the rotational speed signal from the reflective optical sensor and sends a pulse width modulation (PWM) signal to the motor driver for rotation speed control. The spy camera is fixed on top of the PMMA framework and focused on the LoD for microfluidic channel monitoring.
The APELLA was characterized to see how it could be used by studying the mixing process of two colored solutions in a LoD. Additionally, the mixing performance of the APELLA was compared with that of a conventional spin stand while using identical parameters. The spy camera was used to observe the mixing solution injection event with a high imaging temporal resolution. Fig. 1. Comparison between a conventional stroboscopic imaging setup and the APELLA. A) Size comparison of two platforms. B) APELLA implements high efficiency and light weight motor-propeller based driving mechanism with a 49.3 mm long lever arm to drive the rotational part. The APELLA integrates a miniaturized spy camera on top of a microfluidic disc for co-rotating camera imaging. The spy camera provides a fixed temporal resolution (30 frames per second) unrelated to the rotation speed. C) The stroboscopic system takes one high-resolution image of microscale channels per revolution. D) The spy camera follows and monitors the channels on the disc.
In summary, the presented APELLA provides: A cost-effective and open-source platform for LoD research and applications. High imaging temporal resolution regardless of rotation speed. Wireless power for support of imaging and other electronic sensors on the disc. Single Arduino microcontroller for total system control. No need for customized printed circuit boards and metal machining parts.

Design files
The APELLA parts were designed using SolidWorks 2022 (Dassault Systèmes, Vélizy-Villacoublay, France) computer-aided design (CAD) software. All the design files are available in IGES format and can be downloaded from the linked Open Science Framework (OSF) file repository.

Arduino code
The Arduino code was programmed with an Arduino integrated development environment. The Arduino code references a tachometer sketch (courtesy of the author is in the code) and a proportional-integral-derivative (PID) controller (PID_v1.h from Arduino library) that provide precise rotation speed and PID motor control. The PID controller has two stages, where we apply the first stage PID (aggKp, aggKi, and aggKd) when the rotational speed deviates significantly from a speed setpoint. Conversely, we use the second stage PID (consKp, consKi, and consKd) when the rotational speed is close to the setpoint. To prevent overshoot when changing rotation speed, it is needed to fine tune the PID parameters based on the rotation momentum of the LoD. Moreover, the code facilitates the communication between the Bluetooth module and mobile devices for speed parameter setting. Furthermore, the code generates a pulse width modulation (PWM) signal to the motor driver (DRV8830) that drives four motors for the APELLA propulsion. Changing the PWM frequency from 490 to 31,372.55 Hz is essential to avoid abnormal current consumption of the DRV8830 while driving the motors. When uploading a modified Arduino code, removing the Qi receiver ground and power from the Arduino Nano board is suggested.

Electronics
Two 205 kHz Qi wireless transmitters transmit 5 V and 0.96 A power via bottom and top Qi receivers to the rotation part of APELLA. A detailed circuit connection is shown in Fig. 3. The bottom Qi receiver powers the Arduino nano, the Bluetooth 2.4 GHz module, the Wi-Fi spy camera controller, and the motor driver. The RPR220 optical sensor provides a zero-point (when passing a reflection tape) signal to the Arduino connected to the motor driver. The top Qi receiver is connected to the input of the motor driver, which has a parallel connection to four motors. The maximum power consumption of APELLA is 9.74 W, <6% of the power consumption in a conventional spin stand (such as Maxon RE35, 90 W) with a stroboscopic imaging system (Polytec BVS-II Wotan, 75 W).

Bill of materials summary
In addition to the cutting components, items were purchased from other sources. An overview of these components, the number required, as well as the cost can be found in the

Validation and characterization
7.1. Propeller driving mechanism characterization Fig. 7 illustrates a comparison of rotation speed control between a conventional spin stand and APELLA. The spin-stand motor displays perfect speed sequence steps. When APELLA starts to rotate from 0 Hz to 1 Hz, a higher speed (1.7 Hz) is used to overcome the rotational inertia. When increasing the rotation speed, there are large overshoots (29.5-70%) below 6 Hz and small overshoots (0.2-1.68%) above 6 Hz. APELLA has a resolution and a maximum rotation speed of 0.1 Hz and 24.5 Hz, respectively. APELLA has the capability to reverse the propeller rotation for deceleration, but its deceleration performance of 3.05 Hz/s is not optimal for siphon valves on LoD. However, since the rotation part has 5 V power source and an Arduino controller, it is possible to add an actuator for an ''air break" feature similar to airplanes. There is room for APELLA to optimize PID parameters and improve aerodynamic design to reduce overshoot and increase the maximum speed.

Mixing efficiency between APELLA and conventional spin stand
Mixing solutions in a microliter scale volume with a low Reynolds number is challenging since turbulence is hard to achieve in these conditions. For LoD systems, an effective mixing method is based on the Euler force [25]. The Euler force is an inertial force that acts in a direction perpendicular to the radial direction of a disc. When the angular velocity of a rotating coordinate system is changed by acceleration or deceleration, the Euler force can be expressed by the following equation: where F E is the Euler force (N), m is the mass in the rotating coordinate system (kg), r is the distance from the center of the rotating coordinate system to the mass (m), x is the angular velocity (rad/s), and t is the time (s). Equation (1) shows that even at the same acceleration, the magnitude of F E differs depending on when r varies. Fig. 8A shows the principle of the Euler-force mixing while the disc rotates in a counter-clockwise direction. The Euler force F Ea at point a is smaller than the force F Eb at the point b since the distance to the center r a is smaller than the r b . The difference in the Euler force generates a vortex that agitates the solution inside a mixing chamber. One can further enhance the mixing efficiency by changing the rotational speed and direction of the disc. Thus, we implement the Euler force mixing to evaluate APELLA. The mixing efficiency quantitation is performed by analyzing the image inhomogeneity inside the mixing chamber [36]. The captured images are converted to an 8-bit grayscale by an image processing software, ImageJ [37]. The standard deviation, s, obtained by the inhomogeneity of the 3 different analysis areas in the mixing chamber is calculated from the luminance histogram as follows.
Where m i is the gradation value and f i is the luminance of the gradation value. In this way, the standard deviation, s, can be used to describe the inhomogeneity of the solution inside the mixing chamber quantitatively. A good mixture of the solution correlates with a small standard deviation value.
The mixing LoD (Fig. 6) has four different arc-lengths (L = 10, 15, 20, and 25 mm) mixing chambers that are filled with four different volumes (40, 60, 80, and 100 ll) of yellow (inside the reservoir) and blue (inside the mixing chamber) solution for comparison of mixing efficiency. When the mixing LoD reaches a speed of 4 Hz, the meniscus at the interphase between the reservoir and the channel breaks, and the yellow solution flows into the blue solution contained in the mixing chamber. After this, the disc is shaken by changing the rotation direction every 180 degrees at a speed of 1 Hz for 160 s to engage the Euler force. We compared the mixing efficiency between the APELLA and a conventional spin stand (RE 35, Maxon motor AG, Sachseln, Switzerland) with the same visualization module (the rotational part of the APELLA was fixed on the spin stand), mixing LoD, and shaking parameters. Fig. 9A-D demonstrate that the difference in overall mixing efficiency between the APELLA and the conventional spin stand is within the variance range. The experiment results show that the Euler force mixing performance of the APELLA is comparable to that of the conventional spin stand. The spin stand has a higher acceleration when changing the rotation direction. This explains why the 10 mm chamber mixing curve in Fig. 9A shows a lower standard deviation than the APELLA in the first 60 s. Additionally, the APELLA takes longer to reverse direction than the spin-stand due to its momentum.
Moreover, the mixing efficiency increases with the length of the mixing chamber, and a more considerable Reynolds number can explain this due to the higher solution volume. Thus the 25 mm mixing chamber reaches a standard deviation value close to 10 after 40 s compared to the 10 mm one, which reaches the same value but after 120 s.

High temporal resolution mixing visualization
The stroboscopic setup takes one image per disc revolution, which has a limited imaging temporal resolution at lower rotational speeds. However, in the case of LoDs, which prefer lower working speed [7] it is essential to have the temporal resolution independent from the rotation speed. To overcome this problem, the spy camera on the APELLA provides a field of view of 32 Â 18 mm with HD (sensor resolution 1280 Â 720 pixels) and high temporal resolution (30 frames per second) imaging, regardless of the rotational speed. Furthermore, the spy camera rotates with the disc increasing the image quality and reducing the complexity compared to the traditional stroboscopic method.
The APELLA could capture the moment the yellow solution entered the mixing chamber (at 4 Hz speed), which is very difficult to capture with the stroboscopic method (imaging interval 200 ms) at the same speed. Fig. 10 shows six images of the mixing captured by the spy camera with an interval of 33 ms. While the yellow solution entered the mixing chamber, a turbulence flow was created simultaneously, as shown in Fig. 10B. From the spy camerá s images, it was possible to calculate the velocity of the yellow solution during its expansion in the mixing chamber, which was found to be 41.7 mm/s. The  microscale color pattern inside the mixing chamber changed rapidly and continuously. As the mixing time increased, the boundary between the two solutions became obscured, and the color of the liquid in the chamber became almost uniform until 200 s (without shaking). Unlike traditional stroboscopic setups, the spy camera on the APELLA can monitor only one mixing chamber at a time. Multiple spy camera modules are needed for simultaneous multi-chamber monitoring. Table 1 highlights the parameters of APELLA in comparison to other LoD systems with imaging function.
In conclusion, the APELLA achieves: Unconventional propellor driving mechanism proof of concept. Low development cost with off-the-self consumer electronics. Closed loop and sequential speed control. Similar mixing efficiency compared to traditional spin stand. A high temporal resolution of mixing process imaging at a low rotational speed. Lightweight, portable size. Uses 6% power compared to traditional spin stand and stroboscopic system.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.